1. Field of the Invention
The present invention relates generally to the field of external cavity laser diode sources and more particularly, the invention relates to efficient high-power, high-brightness, multi-wavelength external cavity laser sources that use simultaneous spatial-filtering and wavelength selection of each laser bar in a stack to brighten the output of the stack.
2. Description of the Related Art
High-power and high-brightness semiconductor laser sources which have high efficiency are desirable for a variety of applications including machining operations such as cutting and welding, as well as optically pumping other lasers. Current high-power diode laser sources are severely limited in their machining capabilities by the low brightness of their output beams. Although their excellent energy efficiency makes high-power diode laser arrays attractive for laser pumping, their low brightness is also a limitation here for certain laser geometries. For example, prior methods of pumping small-diameter rod lasers and fiber lasers required the use of specialized and expensive optics to obtain reasonable coupling efficiencies.
External cavities have been used to brighten the output of broad-area diode lasers. These have been relatively low power devices with outputs of a few Watts. In contrast, diode laser bars and stacks of bars are high-power sources with outputs ranging from about 30 Watts up to a few kilowatts. If their outputs can be similarly brightened without significantly decreasing output power, these devices should find additional demand, not only in the areas given above, but also in new applications.
U.S. Pat. No. 4,995,050, issued to R. G. Waarts et al., discloses a diode laser external lens cavity configuration having a stripe mirror with two thin parallel stripes placed in front of the two lobes of the array""s far field output pattern. The configuration includes a diode laser array or broad area laser, a lens system, such as a graded-index lens, disposed in front of the laser""s front light emitting facet and the stripe mirror disposed in front of the lens system at the focal plane of the lens. The two stripes are parallel to one another on opposite sides of and equidistant from a vertical reference plane through the lens"" center axis. One stripe is highly reflective, while the other is effectively only partially reflective having either a lower stripe reflectivity or shorter length than the first stripe. Other embodiments include a third stripe spaced from and collinear with the second stripe to form an etalon, and a grating in the cavity. Although the ""050 patent involves spatial-filtering the output of a bar, it does not address the problem of combining the outputs of multiple bars.
U.S. Pat. No. 6,327,292 B1, issued to Sanchez-Rubio et al., discloses a free space external cavity multi-wavelength laser that includes a free-space external cavity and a two-dimensional laser array. The two-dimensional laser array includes a plurality of optical gain elements where each optical gain element generates optical radiation having a unique wavelength and a unique free space optical path. The laser also includes a two-dimensional dispersive optical system that spatially and angularly overlaps each of the free space optical paths in two dimensions. The laser also includes a partially reflecting element that is positioned to intercept each of the free space optical paths. The partially reflecting element, a reflector on each gain element, and the two-dimensional dispersive optical system together form a free space laser cavity that defines the unique wavelengths. The partially reflecting element transmits an overlapping beam comprising radiation having the unique wavelengths. The ""292 does not involve trying to simultaneously brighten the individual sources.
The present invention is an optical system for improving the brightness of a stack of lensed diode bars. It includes a diode bar assembly comprising a stack of lensed diode bars. Front faces of the diode bars have anti-reflection coatings thereon. The diode bar assembly provides an output along an optical axis. The output comprises a plurality of modes in the slow axis, and is collimated in the fast axis and diverging in the slow axis. A focusing optic is positioned so that the front focal plane thereof is coincident with the front faces of the stack of lensed diode bars. A far field pattern comprising primarily two focused spots on opposite sides of the optical axis is produced in the back focal plane of the focusing optic for each of the modes in the output of the diode bars. The spots correspond to the first and second predominant lobes in the mode""s output pattern. A spatial filter is positioned in the back focal plane for discriminating against modes outside a selected modal region of N0 by passing the radiation in the first lobe of the output from the selected modal region of N0. A collimating optic collects radiation passed by the spatial filter and collimates the radiation from the first lobe of the selected modal region. The collimating optic is positioned so as to form, in combination with the focusing optic, a telecentric relay, so that a filtered image of the stack of lensed diode bars is formed at a back focal plane of the collimating optic. A settable grating array comprises a plurality of gratings. The number of the gratings is at least as large as the number of the lensed diode bars. A filtered image of each of the lensed diode bars falls on an associated grating, the angle set for the grating determining which wavelength in the filtered image is returned back through the spatial filter into the lensed diode bar from which it was emitted. An output pickoff element is positioned between the focusing optic and the back focal plane of the focusing optic for turning light from the second lobe of the radiation pattern from the selected modal region into an output path. The light converges toward a focused spot in the back focal plane of the focusing optic before being turned. An output collimating optic in the output path receives the turned light and producing an array of spatially separated, collimated output beams of the turned light. The radiation in each of the beams comes from a corresponding diode bar in the diode laser stack. A wavelength combining assembly receives as input the spatially separated collimated output beams from the output collimating element and overlaps them to form a single, multi-wavelength, collimated beam possessing substantially the same cross section and divergence as an individual input beam. Thus, the stack of laser bars are brightened by simultaneously spatial-filtering each laser bar to brighten its output and setting its spectral wavelength so that it can be spectrally combined with the outputs of the other laser bars.
The invention utilizes a stack of diode laser bars and external resonator optics that act to brighten the output of the stack in the along-junction direction. In the direction perpendicular to the junction in a laser bar (the fast axis) the active region is so thin (sub-micron) that only one mode is typically supported. In the along-junction direction (the slow axis), on the other hand, the power is generally distributed among thousands of modes, producing a radiation output pattern with a divergence more than a thousand times diffraction-limited. Despite the high power obtainable from a single bar (which can be 100 W or more), the excessive divergence (also referred to as the low brightness) of the output can make it unsuitable for many direct applications such as welding and cutting. The low brightness of such laser sources also makes it more difficult to use their output as a pump for some solid state laser geometries.
In this invention the output face of each laser bar is antireflection coated so that it no longer defines the laser cavity. Instead, each bar is placed in an external cavity, which 1) by discriminating against unwanted modes, feeds the power in a much reduced number of modes back into the active medium to brighten the output of the bar, and 2) selects the lasing wavelength of each bar so that the beams from each bar in the stack can be overlapped in space through an external wavelength combining assembly. The greatly improved brightness of the output of a stack of laser bars provided by this invention simplifies their use as pump sources and increases their utility for applications, such as cutting and welding, that require a beam which is both bright and has high power.
Other objects, advantages, and novel features will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.